1. Field of the Invention
The present invention relates to a roll stand for rolling strip. The roll stand includes work rolls which rest against back-up rolls through intermediate rolls, wherein the work rolls and/or the back-up rolls and/or the intermediate rolls are arranged so as to be axially slidable relative to each other, and wherein at least the work rolls and/or the intermediate rolls have a curved or cylindrical contour extending over the entire roll length.
2. Description of the Related Art
When rolling strip of different widths, it is usually necessary for achieving the desired planarity to adjust the crown of the work rolls which is responsible for the planarity. The change of the crown is effected in such a way that the crown of the work roll must be increased with decreasing strip width. This is a result of the increasing supporting effect of the intermediate rolls and the back-up rolls next to the strip edges of the rolling stock which cause the work rolls to be bent adjacent the strip edges; this can only be compensated by a corresponding increase of the crown of the work rolls when the strip width decreases.
Bending of the work rolls causes a thickness reduction of the rolling stock in the areas of the strip edges and an attendant edge sharpening of the rolled strip. In addition to bending of the work rolls, also responsible for this effect is the decrease of the elastic deformations of the work rolls in the unloaded areas next to the rolling stock caused by the rolling load. The better the friction is between the rolls and the rolling stock and the thinner the work rolls are, the more sudden this thickness change of the strip takes place. The drop in thickness of the strip edges increases with increasing strength of the rolling stock and increasing diameter of the work rolls.
In accordance with the prior art, axially displaceable rolls are used for meeting the various requirements with respect to the crown of the rolls for the planarity of the strips and for avoiding the sharpening of the strip edges, so that the entire crown range is covered by only one roll or a pair of rolls.
For example, EP 0 049 798 B1 proposes to provide the work rolls with a curved contour which extends over the entire length of the bodies of both rolls and has a shape in which the two body contours complement each other exclusively in one relative axial position of these rolls. This makes it possible to influence the shape of the roll gap, and, thus, the cross-sectional shape of the rolled strip just by slight displacement distances of the rolls having the curved contours and to also influence the edge pressure and to reduce the edge pressure for preventing the sharpening of the edges.
However, a treatment of the entire strip takes place simultaneously with this influence on the strip edges. For rendering the load distribution uniform, EP 00 91 540 B1 proposes to provide the work rolls as well as the back-up rolls and possibly the intermediate rolls over the entire body length thereof with a bottle-shaped, S-shaped contour which is composed of a convex portion and a concave portion and has a shape where the body contours of rolls which rest against each other or interact complement each other exclusively in one specific relative axial position of these rolls (CVC technology). All rolls are arranged so as to be axially displaceable. Consequently, by a relative axial displacement of the rolls of any of the roll pairs, the contour of the roll gap can be sensitively influenced. However, this known roll stand also still has the disadvantage that the planarity of the strip and the edge pressure are influenced simultaneously.
DE 36 37 206 C2 describes another solution for compensating the disadvantageous effect of roll bending in connection with a four-high rolling mill. The proposed solution is to construct a back-up roll in such a way that the undesired deflections produced by the rolling load are automatically compensated by providing the back-up roll with a solid axial internal portion in the form of a core shaft and an outer circumferential portion in the form of a sleeve, wherein generally cylindrical, outwardly open hollow spaces are arranged between the internal portion and the outer circumferential portion. As a result of this measure, the free ends of the sleeve are bent under the rolling load in a direction opposite to the direction in which they would normally bend if no hollow spaces were provided, wherein bending takes place by a distance which is essentially equal to bending of the core shaft, so that the contact surface between the work roll and the strip material remains essentially flat.
A measure which acts even more directly is described in a not prepublished patent application (Application No. 19626565.7). In order to achieve a uniformity of the strip thickness in the edge areas, the work rolls are provided with notches which extend concentrically about the axes of the work rolls, which results in a different flattening behavior of the roll body provided with the notches as compared to the adjacent solid roll body. A greater flattening takes place at the strip edges as a result of the lacking inner support due to the notches or hollow spaces which cause the strip thickness to be rendered uniform in the width direction of the strip.